here: http://docs.python.org/library/turtle.html#demo-scripts you say that chaotic behavior "proves that you cannot trust a computer's computations" well, the opposite is the case. it proves you can trust computations absolutely. the computations are deterministic. they are correct. that's how the lorenz attractor was discovered. you do need to know what a computer does (and to what) when it computes, and what it doesn't, and how to interpret it. it's misleading to say that computation is untrustworthy. computation is a particular thing, that's all. it might seem pedantic but I think it's a much more interesting and important thing for a student to think about than "untrustworthy". which is just false anyway. as you know I'm sure. I'm not trying to be patronising.